yea as i understand it the vehicles speed is read from the ringgear.. not from the trans output like older vehicles..

tires size def. affects your reading but not gearsets.

 

Don't overlook JL's advice about load, especially when towing.

The best mileage I ever got in my 99 was a run between Amarillo and Albuquerque at 85 mph. My rpm's hovered around 3000 the whole way with my 4:30's and I still got 13.5 mpg.

A revving engine works easier and the gas converting to energy is most efficient at the torque peak rpm.

 

After spending a lot of time reading here, I ordered my second V10 truck (my current one) with 4.30 gears. I've never regretted it. If you ever plan on going to a 35-37" tire, get 4.56s.

 

if you want to tow between 55-60 and pulls medium hills in OD I would go with 4:56 If you want to pull those same hills at 65 or above 4:30's

 

That doesn't make sense. It would be easier to tow at 65 or higher with the 4.56, not the 4.30. It would be even easier to do it with the 4.88 gear.

 

No,it does-think about it.
At higher speeds,the engine is at higher RPM's,and closer to the peak torque.
JL

 

Which would support my statement and not his since the peak power is over 3k rpms.

 

That great mileage is because you have a tailwind 90% of the time! I take the road 4 times a year, try checking your mileage from Amarillo to ABQ.

Go with the gears, 4.30 with your current wheel/tire combo. It won't change the speedo.

 

I don't remember if there was a tailwind or not. But what I do remember is I was heading west and climbing in altitude the whole way it seemed.

That was one of the very few times I drove the '99 that fast for that many miles. West Texas and eastern New Mexico is like the autobahn to me.

 

I had a 00 Ford Excursion V-10 with 4.30 gears (factory) and I was able to get 15 sometimes 16 mpg with the cruise set at 62 mph on flat expressway empty not towing. Towing my 9K TT I was getting 7 mpg-9 mpg depending on conditions. I found at 70 mph with OD on the engine seemed to be right in its torque band and I could handle small hills without downshifting. Now I have a E350 V-10 with the 3.73 gears and towing the same trailer it has to be relatively flat to hold OD and with any wind or incline I have to take it out of OD.

 

the 4:56"s gears will get you into that peak power at a slower speed because you are turning more rpm's

 

Peak is over 3,000 rpms so you could use 4.88s and still not be at peak power in OD until over 80 mph on stock tire.

 

that may be the case but no matter how you look at it the 4:56 gear will turn more rpm at a slower speed then the 4:30 making it easier to pull in overdrive at a slower speed,with my truck and (camper-9000lbs) I can pull up over pretty decent hills in overdrive at 65mph if I drop down to 60mph then the truck shifts out of overdrive,now if I had the 4:56 gears (which I wish I had)I should be able to pull that same hill in overdrive but at 60 mph because the deeper gear would turn the same rpm as the 4:30 needed to hold overdrive at the lower speed.I never mentioned anything about which gear would be running at peak power in overdrive because that may not always be possible in overdrive.PLAIN AND SIMPLE THE 4:56 WILL HOLD OVERDRIVE AT A SLOWER SPEED BECAUSE IT'S TURNING MORE RPM ok I'm done with this now, back on topic

 

With my 07's 4:30's and 285 tires I am turning 3k at 80 mph.

 

In my '99 Scab 2 WD stock tires, I changed the 373's for a 430 Posi Rear end. I debated 410's vs 430's but I'm very glad I went for the 430's. While the RPMS are slightly higher especially when not towing there isn't a big hit in mileage. Towing, the 430's hold OD longer and thus I get better milleage with the new gears over the old. I tow a 7500# travel trailer. FWIW my speedometer reads correctly after the change. I also had a tranny temp gauge installed at the same time so I could keep an eye on it when towing.

If you are going with bigger tires I would consider 456's, but for me and my purposes, the 430's work very well.